As an apparatus for indicating the electromotive force of a dry battery, a device employing a liquid crystal is known (For example, U.S. Pat. No. 4,006,414). The apparatus (hereinafter referred to briefly as a battery checker) of this type is based on the principle that the liquid crystal heated by an electrically conductive resistance element adapted to generate heat on contact with the electrodes of a dry battery changes its optical property in response to the temperature change caused by the generated heat to undergo visible coloration or change in color, thus enabling to ascertain the residual energy of the dry battery which is represented by electromotive force. However, this liquid crystal device has much to be improved, as indicated below.
(a) Since the developed color or the induced change in color is not distinct, it is impossible to accurately estimate the electromotive force.
(b) Since the color varies according to the viewing angle, a variation in results of measurements is inevitable.
(c) Since the change in color indicating the electromotive force is monotonous or the variety of developed colors is very limited, the device is not suited for use by people with dyschromatopsia.
(d) Since color variation temperatures are fixed for a given kind of liquid crystal, it is impossible to freely adjust the combination of color variation temperature with color developed.
(e) Since the liquid crystal cannot be mixed with pigments, dyes or the like, the color tones cannot be modified.
(f) The device is expensive.
(g) Liquid crystals are generally poor in light fastness and their light fastness cannot be improved because ultraviolet absorbers and the like cannot be incorporated.